Maybe the sponsors can post this as a sticky as I have been replying to a lot of these hard start threads, which always happens when it starts to get cold.
Here goes and I hope it helps.

How to check Glow Plug System

To check the Glow Plug Relay (GPR)
∑ Be sure the engine is cold, so that the PCM will tell the GPR to turn on. If the engine is hot, you wonít have as much time to check.
∑ Locate the GPR Ė Its behind the fuel filter on top of the engine, a little bit toward the passenger side of the valley. There may be two relays there. If so, the rear one is the GPR. It will have two fairly large wires (yellow and brown) connected to one of the large posts.
∑ With your multitmeter set to DC volts, and 15 V range (if not autoranging), clip the positive (red) lead to the output terminal (with yellow and brown wires connected), and the negative (black) lead to a good ground point (like the battery ground terminal or someplace metal directly on the engine block.)
∑ Turn the key to ON (do not start)
∑ If your GPR is good, it should click, and youíll see 11 volts or so on your meter, then, depending on temperature, it will click off up to 2 minutes later. You should do this a couple of times to make sure it consistently makes the connection.
∑ If you donít get voltage with this test, confirm by retesting as follows.
∑ Remove the two small wires from the smaller two of the four GPR terminals.
∑ With jumper wires, apply voltage from the battery across the two small terminals. If your voltmeter now reads voltage on the output terminal, your GPR is OK, and your problem is in the PCM circuit that tells the GPR to activate.

If your GPR is bad you can use the factory replacement for around $75, Napa's GP110 is close to this price maybe $10 cheaper. But you can get a GPR 109 from Napa for around $22.00 This is the same exact relay as the GP110 except the mounting holes are rotated 180 degrees, which is no big deal as the wires stretch just fine.
Now if you are tired of replacing your GPR and want a H.D. alternative may I suggest the Stancor 586-902. This is a large relay and it can truly handle the large AMP draw our trucks call for at start up. Gopher Electronics has these for under $40. I know several folks that live way up North (Alaska, Canada) where they know about serious cold starts and they all swear by the Stancor. I am very happy with mine, I believe I have pics of mine installed in my webshots.

To check Glow Plugs.
∑ Remove the electrical connector on the inboard side of valve cover at the gasket. Press down on the top of the connector latch and pry gently with a screwdriver. Photo of disconnecting one and another Photo of it loose.
∑ There will be 9 pins on the valve cover gasket where you removed the connector. The two pins furthest forward and the two pins furthest back are for your glow plugs.
∑ With your multimeter set to resistance (ohms) and low range (single digits) if not autoranging, clip the negative (black) lead to a good ground point.
∑ Probe each of the 4 outer pins individually with the positive (red) lead, noting the resistance. Good glow plugs will have a resistance between 0.6 and 2 ohms. If you get infinite resistance on any glow plug, that one is either bad or the connector under the valve cover has come loose.
Take Care
Kevin

Also let me add, that if your GPR does test pad you can use a Napa model #109, their book will call for the #110 but the 110 is around $60+ where the 109 is low $20's. They are the same exact sensor spec wise, the only difference is the mounting tabs are rotated 180 degrees, don't worry the wires make it fine. However if you really want a HD relay and one you won't have to change yearly, you should replace it with the HD Stancor unit, the model # is 586-902 and it is made to handle the 200 amp draw, whereas the others are made very cheaply. You can see photos of this relay in my webshots.

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